Mycobacterium tuberculosis is frequently called "the most successful" human pathogen, owing to the fact that when an individual has been infected by the bacteria, one very rarely eliminates the infecting organism from their system. M. tuberculosis can persist in humans only to become reactivated later under immune compromised situations that disrupt the equilibrium of the host immune system as it interacts with the infecting microbe, such as co-infection with HIV or treatment with anti-inflammatory drugs for rheumatoid arthritis. Therefore, one of the most important problems in understanding TB is that the bacteria possess very effective mechanisms of evading elimination by adaptive immunity. In order to control an infection with M. tuberculosis the host immune system must generate an adaptive immune response, and without such a response, patients succumb to an overwhelming M. tuberculosis infection. Effector CD4 T cells are especially important for the control of a TB infection, however, they are not sufficient to eliminate bacteria from the host. M. tuberculosis evading or disrupting T cell activation and recognition in the host, could explain the persistence of this pathogen within its host;this makes further studies of T cell responses to Mycobacterium of particular importance. In animal models of infection with M. tuberculosis and M. bovis BCG (an attenuated strain of mycobacterium), both show similar kinetics of adaptive immune activation;however, in contrast to M. tuberculosis, the immune response to M. bovis BCG results in the eradication of the bacteria from its host. We hypothesize that the dichotomous outcome of infection between these two strains of mycobacterium are the result of a differential activation of effector T cells. We will use the techniques of confocal and two-photon microscopy to assess and compare the frequency and quality of antigen-specific CD4 T cell recognition of mycobacterium infected cells in the lung. By identifying differences in the frequency and the quality of T cell interactions with M. tuberculosis and M. bovis BCG infected cells, we will further understand the breakdown in T cell instruction that occurs during M. tuberculosis infection and allows the pathogen to evade effector T cell recognition and persist within its host. PUBLIC HEALTH RELEVANCE: The study of T cell responses to M. tuberculosis represents a critical step towards the development of vaccines to protect against tuberculosis. We aim to understand the dynamics of T cell interactions with M. tuberculosis infected cells within lung tissue to elucidate mechanisms employed by the pathogen to evade eradication. An understanding of these key mechanisms will assist in the efforts to design novel vaccines that more efficiently target and activate effector T cells to protect humans from tuberculosis.